Stereoisomerism: Z/E, asymmetric carbon, enantiomers

Stereoisomers have the same molecular formula and the same bonds, but a different arrangement in space. Conformers differ only by rotation about a $\ce{C-C}$ single bond: they are the same molecule and cannot be isolated. For butane, the staggered anti conformation ($\ce{-CH3}$ groups at $180^\circ$) is the most stable, ahead of gauche ($60^\circ$) and the eclipsed ones.